1. Field of the Invention
The present invention relates to a method for conveying a cut sheet and to a sheet conveying apparatus controlled based on the sheet conveying method.
2. Description of the Related Art
In general, a printer apparatus conveys a printing sheet from a cassette (i.e., a sheet container) to a printing unit. As discussed in Japanese Patent Application Laid-Open No. 2005-306605, a printer apparatus can change the orientation of a sheet being separated and fed from a cassette by an angle of approximately 90 degrees before the sheet is conveyed to a printing unit.
The sheet conveying apparatus discussed in Japanese Patent Application Laid-Open No. 2005-306605 is simply described below with reference to FIGS. 28A to 28F, 29A to 29F, and 30.
In FIGS. 28A and 29A, a recording sheet supporting plate 104 shifts downward to press recording sheets against a sheet feeding roller 103. The sheet feeding roller 103 rotates in the clockwise direction so that a bottommost recording sheet can be conveyed rightward. A recording sheet positioned next to the bottommost recording sheet does not receive a pulling force from the sheet feeding roller 103. A separation friction member prevents the next recording sheet from being conveyed together with the bottommost recording sheet.
A recording sheet P, having being conveyed rightward, stops at a position where a leading edge of the sheet P reaches an orientation changing roller pair 105, whose rollers are in a separated state.
The orientation changing roller pair 105, as illustrated in FIG. 30, includes a pair of driving rollers 105a and 105b and a pair of driven rollers 105c and 105d, between which a recording sheet can be held. The driving roller 105a is independently driven by an orientation changing motor M1. The driving roller 105b is independently driven by an orientation changing motor M2.
The orientation changing motors M1 and M2, in a forward driving mode, rotate the driving rollers 105a and 105b in a clockwise direction. The driving rollers 105a and 105b cooperatively convey a recording sheet in a predetermined feeding direction. In this condition, the orientation changing roller pair 105 keeps rotating so that the bottommost recording sheet can be continuously conveyed in the sheet feeding direction and completely removed from a recording sheet container.
As illustrated in FIGS. 28B and 29B, a front half of the recording sheet P protrudes out of the apparatus body via an aperture 113.
Then, as illustrated in FIG. 29C, the sheet conveying apparatus adjusts the position of the recording sheet P in such a way that the orientation changing roller pair 105 can hold substantially the center of the recording sheet P in the sheet conveyance direction. To this end, the orientation changing motors M1 and M2 rotate in a forward direction or in a backward direction so that the distance between a leading edge of the recording sheet P and the holding point of the orientation changing roller pair 105 (i.e., D1 indicted in FIG. 29C) becomes equal to the distance between a trailing edge of the recording sheet P and the holding point of the orientation changing roller pair 105 (i.e., D2 indicated in FIG. 29C).
According to the illustrated example, the orientation changing motors M1 and M2 rotate in the backward direction so that the orientation changing roller pair 105 can hold substantially the center of the recording sheet P in the sheet conveyance direction. Thus, the recording sheet P is conveyed in a direction opposed to the sheet feeding direction and stops at the position illustrated in FIG. 29C.
Subsequently, as illustrated in FIG. 29D, the sheet conveying apparatus changes the orientation of the recording sheet P. In this case, rollers of a conveyance roller pair 106 are kept in a separated state. The orientation changing motors M1 and M2 drive the driving rollers 105a and 105b of the orientation changing roller pair 105 so as to rotate in mutually opposed directions as illustrated in FIG. 30. The driving rollers 105a and 105b, rotating in mutually opposed directions, can change the orientation of the recording sheet P in the interior space of the apparatus.
More specifically, the driving rollers 105a and 105b rotate a longer edge of the recording sheet P by 90 degrees relative to a direction perpendicular to the sheet conveyance direction. The driving rollers 105a and 105b stop rotating when the recording sheet P completes a rotating motion of approximately 90 degrees.
In the process of changing the orientation, the recording sheet P may reach the conveyance roller pair 106. The rollers of the conveyance roller pair 106 are required to be kept in a separated state. However, if the recording sheet P does not reach the conveyance roller pair 106 in the process of changing the orientation (i.e., when the dimension K indicated in FIG. 28D is sufficiently long), the rollers of the conveyance roller pair 106 can be kept in a pressed contact state.
Furthermore, in the process of changing the orientation, the recording sheet P causes a rotating (yawing) motion in a state where the recording sheet P partly protrudes out of the apparatus body via the aperture 113.
FIG. 29E illustrates the recording sheet P shifting leftward before being subjected to image forming processing. The sheet feeding roller 103 and the recording sheet supporting plate 104 can be kept in any state. At least one of the orientation changing roller pair 105 and the conveyance roller pair 106 is kept in a pressed contact state. In this state, the rollers of the orientation changing roller pair 105 or the conveyance roller pair 106 convey the recording sheet P to an image forming unit 120. The sheet conveying apparatus stops the above-described conveying operation when the leading edge of the recording sheet P reaches the image forming unit 120. In this case, either the orientation changing roller pair 105 or the conveyance roller pair 106 can convey the recording sheet P.
Furthermore, the sheet conveying apparatus performs a skew correction in such a way that the orientation of a side edge of the recording sheet P becomes parallel to the sheet conveyance direction. As illustrated in FIG. 29E, driven rollers 106b of the conveyance roller pair 106 are obliquely disposed relative to the sheet conveyance direction.
When the conveyance roller pair 106 conveys the recording sheet P along the conveyance path, a side edge of the recording sheet P is kept in contact with a reference wall 108 (i.e., a projection provided in the conveyance path). As a result, the side edge of the recording sheet P becomes parallel to the sheet conveyance direction. Such obliquely disposed rollers are generally referred to as “skewed rollers.”
The technique for regulating the position/direction of a side edge of a recording sheet is discussed in Japanese Patent Application Laid-Open No. 8-208075 and Japanese Patent Application Laid-Open No. 7-334630.
When the driven rollers 106b of the conveyance roller pair 106 serve as the skewed rollers, the orientation changing roller pair 105 is kept in a separated state and does not interfere with the skewed rollers, which regulate the side edge of the recording sheet P shifting in parallel with the sheet conveyance direction.
As illustrated in FIG. 28F, the image forming unit 120 prints an image on the recording sheet P. Then, the sheet conveying apparatus discharges the recording sheet P from a sheet discharge port 114.
The above-described sheet conveying apparatus can reduce the body size, although a complicated mechanism for rotating a sheet is required.
However, according to the above-described sheet orientation changing method discussed in Japanese Patent Application Laid-Open No. 2005-306605, the rotating motion of a sheet tends to deteriorate sheet positioning accuracy.
FIG. 31 illustrates an exemplary operation of the above-described conventional sheet conveying apparatus that conveys the sheet illustrated in FIG. 29C straight and stops the sheet at a position where the rotation center of the sheet accords with a predetermined rotation center set in the apparatus.
Although not described in Japanese Patent Application Laid-Open No. 2005-306605, the control for positioning a sheet advancing straight can be performed based on a closed loop control using a sheet detecting sensor provided on a sheet conveyance path.
In FIG. 31, “A” indicates a recording sheet having been conveyed accurately without causing any skew, and “B” indicates a recording sheet inclined relative to the correct orientation. For example, the sheet conveying apparatus stops the sheet based on a closed loop control using a sensor 140 as illustrated in FIG. 31. The distance L from a rotation center C to the sensor 140 is not different between the sheet A and the sheet B, which are stopped at the position being detected by the sensor 140.
FIG. 32 illustrates a sheet rotating operation performed by the sheet conveying apparatus. In FIG. 32, “R” represents a rotation radius of the sheet A (i.e., the distance from the rotation center C to the edge of the sheet A), and “r” represents a rotation radius of the sheet B (i.e., the distance from the rotation center C to the edge of the sheet B). The rotation radius “r” of the sheet B is smaller than the rotation radius “R” of the sheet A.
The orientation changing roller pair 105 does not include any mechanism for correcting the difference between the rotation radii R and r. FIG. 33 illustrates a positional deviation R-r between left edges of the sheet A and the sheet B. The orientation changing roller pair 105 cannot shift the sheet in the right-and-left direction. Therefore, the positional deviation R-r caused in this manner may deteriorate sheet positioning accuracy. In other words, the above-described conventional sheet conveying apparatus has inaccuracy in positioning a rotated sheet.
As a method for regulating the left edge of a rotated sheet, it is useful to provide the “skewed rollers” that can pull the sheet in the right-and-left direction and convey the sheet along a reference plate provided on the sheet conveyance path, as discussed in the above-described Japanese Patent Application Laid-Open No. 8-208075 and Japanese Patent Application Laid-Open No. 7-334630.
However, the mechanism including the “skewed rollers” requires a significantly long conveyance path for completely regulating the position/orientation of a sheet while the sheet is conveyed along the sheet conveyance path. Thus, the “skewed rollers” cannot be used for a sheet conveying apparatus that is required to reduce the body size.